A conventional heavy duty slab shear is illustrated in FIGS. 1A and 1B. This shear is a hydraulic up-cut type shear in which the bottom blade 1 supported on bottom beam 4 is moved upwards by two cylinders 5 in order to carry out the shearing action. The shear features a single raked top blade 2 at a fairly steep rake, typically 16:1, in order to keep the load at a reasonable level when cutting thick, hard alloy slabs, typically of the order of 150 mm thick. The design illustrated in FIGS. 1A and 1B also features a toggle lowering and raising mechanism 14 which is operated by hydraulic cylinder 15 for quickly lowering the top blade to a fixed position for the cut and raising it to provide a large clearance. As is usual with such toggle mechanisms it is designed such that when the top blade is in the lowered cutting position the links are very close to vertical and therefore there is almost no load on the cylinder 15 and the cutting force is transferred directly through the links to the housing. The hold down clamp has been omitted from FIGS. 1A and 1B for clarity.
The conventional design illustrated in FIGS. 1A and 1B also features a torque tube and linkage mechanism 10, 11, 12, 13 to keep the bottom beam 4 and bottom blade 1 level during the cutting movement. This type of mechanism is well known in the prior art. For example U.S. Pat. No. 2,699,649 features a similar torque tube and linkage arrangement. The advantage of using such a torque arm linkage mechanism is that each of the cylinders 5 only has to produce approximately half of the total cutting force.
Although the shear functions satisfactorily, these features result in certain operational problems. Because the rake of the top blade is chosen to be quite steep in order to minimise the cutting force when cutting thick and strong material it causes significant distortion and twisting of the slabs and plates at all thicknesses with resultant impact on roller tables and potential issues for getting plate products into the jaws of the stretcher. Also the bottom blade has to move upwards by the same large distance for all products in order to complete the cut and therefore all the products drop back onto the table by this distance and this can cause marking and damage. In addition, the cylinders 5 have to move through a large stroke for every cut which requires a lot of hydraulic oil and which makes the cutting time longer than it needs to be.
In addition, increasing width and duty requirements are stretching the capability of the existing shears and the use of a rolling blade shear solution, whether hydraulically operated as in GB2405118 for example or mechanically operated as in EP1572408 for example is costly. Particularly, when shearing aluminium, flexibility is important, so shears are required to perform the mixed functions of nose cropping, tail cropping, dividing and plate shearing, not just a single function.
When cutting thick and hard material it is clearly advantageous to have a fairly steep rake on the blade in order to minimise the cutting force. However, when cutting thinner material it is advantageous to reduce the rake of the blade in order to minimise the distortion and twisting of the material, to minimise the cutting stroke required and thus reduce the cutting time and—in the case of an up-cut shear—to minimise the distance which the cut piece has to drop back onto the table.
U.S. Pat. No. 3,568,558 describes an arrangement for controlling the angle of inclination between an upper movable blade and a lower fixed blade of a shear. A pulley system operates a pair of pilot valves to adjust the angle of the moveable blade and to move the moveable blade up and down at the predetermined angle. This arrangement is capable of setting a steep rake when cutting thick and strong material and of setting a reduced rake when cutting thinner material. However in this arrangement each of the two hydraulic cylinders 2 and 3 must be capable of generating the full cutting force or almost the full cutting force. The reason for this is that, when shearing with a raked blade, at any particular instant during the cutting cycle the cutting force is only applied to a relatively short length of the blade and the position of the highly loaded part of the blade moves along the blade during the cut. By taking moments of the forces it is clear that when the loaded part of the blade is at one end almost all of the cutting force is applied to just one of the cylinders.
U.S. Pat. No. 3,872,756 describes an alternative mechanism for controlling rake angle and stroke of a shear having an upper movable blade and a lower fixed blade. This design suffers from the same problem as that of the shear described in U.S. Pat. No. 3,568,558; namely that each of the hydraulic cylinders 27 must be capable of generating the full cutting force or almost the full cutting force.
In these prior art shears the fact that the two hydraulic cylinders each have to be capable of generating the full cutting force makes the shears expensive to construct and also means that they require approximately twice the hydraulic fluid flow that is theoretically required.